Data communication via infrared radiation is disclosed in the co-pending application INFRARED DATA COMMUNICATIONS, Ser. No. 184,261. That application describes an invention for an apparatus for wireless communication of information between two locations. A transmitter means modulates and transforms the information into infrared radiation at a first location. The infrared radiation is transmitted by the transmitter to a reflecting surface along a straight line path which is transparent to the infrared radiation. The infrared radiation is diffusely reflected at the reflecting surface forming reflected infrared radiation. A receiver means for receiving the reflected radiation from the reflecting surface is positioned at a second location. The second location and the reflecting surface have a straight line path there between which is transparent to the infrared radiation.
In a second co-pending application INFRARED NETWORK TRANSCEIVER APPARATUS, Ser. No. 271,570, an apparatus is disclosed for an illumination transmitter. The transmitter projects a signal onto a planar reflective surface such as the ceiling of an open office environment. The transmitter includes a plurality of illumination sources positioned in a line wherein the line has a predetermined orientation to the planar surface. The illumination sources are preferably infrared LED's. The transmitter has means for jointly aiming the illumination sources toward a desired location on the planar surface. The location on the planar surface is usually used to allow multiple transmitters coupled to infrared receivers to allow communication between computers in a LAN. The transmitter also includes means for maintaining the predetermined orientation between the planar surface and the line. The orientation is usually parallel to allow transmitters located large distances from the illumination to project a relatively small illumination spot onto the planar surface.
In a LAN there are a plurality of distributed computers, terminals and peripherals. In a hard wired LAN, collision detection and avoidance relies upon the fact that each resident on the network can detect every other network resident. In an infrared network it is possible that not every resident on the network can communicate with every other resident. FIG. 1 shows a situation where three network residents 20, 22 and 24 are located within a room 26. Transceiver 20 is positioned across the room from its projected spot 28 on the ceiling. Because the illumination spreads out as it travels the illumination projection 28 for the transceiver 20 can be relatively large. In contrast, transceivers 22 and 24 direct their infrared illumination almost straight up toward the ceiling forming smaller illumination projections 30 and 32, respectively. Each of these smaller illumination projections 30 and 32 fall within illumination projection 28. Accordingly, the LAN resident at transceiver 20 (transceiver 20) can communicate with both transceivers 22 and 24. However, as indicated in FIG. 1, the illumination projections 30 and 32 do not overlap. This prevents the transceivers 22 and 24 from communicating with one another over the infrared network.
If transceiver 22 and transceiver 24 are unaware of one another, it is possible that both these transceivers 22 and 24 could attempt to transmit information to transceiver 20 at the same time. This results in a transmission collision so that transceiver 20 merely receives two messages at the same time and results in incorrect reception of information. This situation does not exist in a LAN employing cable.
A method is needed for properly aligning the illumination projections of infrared transceivers into a network to establish communication among all transceivers. The method and apparatus should include means to prevent collision transmission as additional transceivers are added to an established network.